Screening apparatus with self-adjusting eccentric weight



w. A. HUNTER 3,160,584

SCREENING APPARATUS WITH SELF-ADJUSTING ECCENTRIC WEIGHT Dec. 8, 1964 2 Sheets-Sheet 1 Filed Jan. 8, 1962 WILLIAM A. HUNTER Eran Er Dec. 8, 1964 w. A. HUNTER 3,160,534

SCREENING APPARATUS WITH SELF-ADJUSTING ECCENTRIC WEIGHT Filed Jan. 8, 1962 2 Sheets-Sheet 2 @5 138 1 a V //V/ /V A 5 I ,W 4154 -\\\\w 36 I I l 152 4 100 l i a 1220 114 116 112 I; 168 1oz EZEZTZIEE WILLIAM A HUNT ER 3,169,584 Patented Dec. 8, 1964 United States Patent Office 3,160,584 SCREENING APPARATUS WITH SELF-ADJUST- ING ECCENTRHCWEHGHT Wiiliam A. Hunter, Morton Grove, BL, assignor to Pettibone Muiliken Corpnration, Chicago, 111., a corporation of Deiaware Filed Jan. 8, 1962, Ser. No. 164,905 3 Claims. (Cl. 209-3665) The present invention relates generally to screening apparatus. More particularly the invention relates to that type of apparatus which is designed or adapted for use in a foundry in connection with the screening of molding sand and comprises a substantially horizontal frame, an inclined screen in the frame, and motor driven mechanism for effecting rapid gyration of the frame and screen in such a manner that the sand to be screened, when brought into engagement with the screen, is subjected to agitation whereby the small or fine particles are caused to pass through the screen.

A present day screening apparatus of the aforementioned character is possessed of certain limitations, both as regards the construction and the operation thereof. One such limitation resides in the fact that the gyratory screen of the apparatus invariably is eccentrically driven on a fixed offset radius and, because of this fact, the counterweight which is always employed to counterbalance the resultant moment of mass, not only of the screen but, additionally, of the load upon the screen, must be designed for a given contemplated load. If the load is to be changed, then the counterweight also must be changed accordingly. Another limitation that is attendant upon the construction and use of the present day screening apparatus, particularly an apparatus of the belt driven type, resides in the fact that the driving center at the driven end of the drive shaft for the screen is not fixed with respect to the driving center of the motor pulley. Although the amplitude 'of oscillation of the driven end or" the drive shaft is considerably smaller than is the amplitude of oscillation of the end which is attached to the screen, it is sufliciently great as to affect the proper operation of the belt, apply undue stresses to the belt and thus shorten its life, transmit vibration to the driving motor, and otherwise raise the general noise level of the apparatus. Yet another limitation of a screening apparatus of the type under consideration is a limitation of design or construction in that the driving means for the screen requires the use of a dual-shaft assembly including a main shaft and an eccentrically mounted shaft. Such a dual-shaft assembly requires the use of a minimum of no less than three bearing assemblies for proper support thereof, two bearing assemblies being required for the main shaft and at least one bearing assembly for the eccentric shaft. Other limitations, such as excessive apparatus vibration, a high noise level, frequent maintenance and the like, are additional factors that leave much to be desired in connection with the construction and operation of present day screening apparatus.

The present invention is designed to overcome the above noted limitations that are attendant upon the construction and use of screening apparatus of the offset gyratory type and, accordingly, it contemplates the provision of an apparatus which loosely may be described as being of the non-eccentric type in that it employs a single I- tary drive shaft, the lower end of which is confined or restrained against oscillatory movement while the upper screen-attached end is capable of limited oscillation. An off-center or eccentric weight which is fixedly secured to and rotatable with the drive shaft presents with respect to the axis of the shaft a moment of inertia which is of such magnitude that the eccentric weight, due to the unbalanced centrifugal forces involved when the shaft is rotated, sets up a gyratory or wobbling motion in the end of the shaft to which the Weight is attached. Disregarding for purposes of discussion the mass of the end of the shaft to which the weight is attached and of the shaftbearings at this end of the shaft, the ofi-center weight, in its free state, presents a definite radius of gyration. The radius of gyration of a body may be defined as the distance from its axis of rotation to a point at which the entire mass of the body may be considered concentrated without altering its moment of inertia. Considering now the mass of the shaft, the bearings and the attached screen, together with the mass of the off-center weight, the radius of gyration of these bodies, considered as a whole, will present a different and definite radius of gyration. Considering still further the effect of a load of molding sand upon the screen. if this load be regarded as a component of the assembly including the weight, the shaft, its bearings and the screen, still another different and definite radius of gyration will be established. Obviously, a quantity of sand, resting on a gyrating screen cannot be considered to have a fixed or definite center of mass or moment of inertia, nor can such a quantity of sand be regarded as a fixed component that can gyrate with the screen in unison therewith. There will be much particle slippage and mass shifting, but the effect of such a quantity of sand will be applied to the screen through frictional and other phenomena so that the greater the quantity of sand, by weight, the shorter will be the amplitude of gyration of the weight, the shaft and its bearings and the screen. Stated in other words, the load which is applied to the screen will affect the'amplitude of gyration of the screen, shortening the amplitude of gyration as the load increases.

From the above discussion, it will be seen that the off-center weight employed in connection with the present invention is in no sense a counterweight but rather it is a gyration or wobble producing element which possesses self-balancing features so that changes in the load upon the screen are automatically compensated for without attention on the part of the operator.

The provision of a screening apparatus of the character briefly outlined above constitutes the principal object of the invention.

The provision of a screening apparatus which is possessed of extreme simplicity of design and, therefore, may be manufactured at a relatively low cost; one which is possessed of a minimum number of parts, especially moving parts, and is therefore, unlikely to get out of order; one which, by reason of the fact that the single drive shaft for the gyratory screen is maintained at a fixed distance from the center of the driving pulley, enables a belt drive to be employed wherein belt tension does not vary and confines the gyratory motion entirely to the screen so that the driving motor, together with the mounting means therefor, is free from induced vibration, thereby contributing toward an apparatus which is smooth in its operation and possesses a low noise factor; one which is rugged and durable and which, therefore will with stand rough usage; and one which, otherwise, is well adapted to perform the services required of it, are further desirable features which have been borne, in mind in the production and development of the present invention.

Numerous other objects and advantages of the invention, not at this time enumerated, will readily suggest themselves from a consideration of the following detailed description.

In the accompanying two sheets of drawings forming a part of this specification, one illustrative embodiment of the invention has been shown.

In these drawings:

FIG. 1 is .a side elevational view of a screening apparatus constructed in accordance with the principles of the present invention, certain parts being broken away and 'other parts being shown in section inorder more clearly to reveal the'nature of the invention;

FIG. 2 is an end elevational view of the apparatus of FIG. 1, looking from the discharge end of the apparatus, with parts being broken away and other parts being shown in section;

FIG. 3 is an enlarged sectional view taken on the line 3-3 of FIG. 1;

FIG. 4 is an enlarged sectional view taken on the line 4-4 of FIG. 2:

FIG. 5 is an enlarged sectional view taken on the line 5-5 of FIG. 2; and

FIG. 6 is an enlarged sectional view taken on the line 6-6 of FIG 1. a

The improved screening apparatus which is shown in the drawings constitutes the preferred form or embodiment of the invention. It is essentially and primarily adapted for use in a foundry or like establishment in connection with the screening of molding sand and embodies as its main or principal parts a screen assembly 10, a

base frame 12, gyrating mechanism 14, and poweractuated driving means 16 for operating the gyrating mechanism 14.

The screen assembly 10 is comprised of three principal parts, namely, a screen proper 20, a screen frame 22 and a hopper 24. These three parts are adapted to gyrate V bodily as a unit about a fixed vertical axis at varying amplitudes, i.e., onvarying radii, in a manner and for a purpose that will become clear presently. The screen frame 22 is generally of open rectangular configuration and is comprised of marginal angle bars 26 across which the screen20 extends. The angle bars 26 are removably seated upon a ledge portion which is afforded by an inside marginal frame 28. The latter is welded or other- I ing asloping bottom wall 34, and two pairs of opposed upstanding side walls 36. The upper margins of the side walls are bent outwards to form a reinforcing flange 38. One pair of opposed side walls slope inwardly as at 40 (see FIG. 2) and .join the side margins of narrow bottom wall 34. At the discharge or lower portion of the hopper 24, one of the side walls is provided with a discharge or sand metering gate '42.

The base frame 12 is adapted to be fixedly supported upon any suitable supporting structure as, for example,

. a framework over a sand pit in the floor of a foundry. 'It

is generally of rectangular configuration and comprises marginal frame members in the form of angle bars 44. As shown in FIG; 1 the small dimension of the base frame 12 is somewhat shorter than the longitudinal extent of the hopper 36 while, as shown in FIG. 2, the long dimension of the frame 12 is somewhat longer than the transverse extent of the hopper. The base frame 12 supports at the sides thereof a series of four upstanding reaction posts 46 which, in turn, serve movably to support the entire gyratory screen assembly 10 so as to permit of a limited amount of gyrating motion thereof while at the same time preventing rotation of the assembly under the influence of the rotating parts of the gyrating mechanism 14. The reaction posts 46 are adapted to swing or tilt laterally as hereinafter described so that they may become displaced slightly and angularly from the normal vertical positions in which they are biased by their mounting means to allow for the gyrating movements of the screen assembly. I

The four posts 46, together with their mounting means, are identical in their construction and, therefore, a description of one of them will suflice for all. Each post 46 is of a composite nature and includes an outer tubular member 48 which is connected at the upper end thereof by a resilient mounting assembly 50 to the screen assembly 10. The lower end of each post is connected by a resilient or flexible mounting assembly 52 to the base frame 12. The lower mounting assembly 52 allows for limited degree of swivelling movement of the associated post 46 through a very small angle, measured only in seconds, while the upper mounting assembly 59 allows for a limited degree of lateral offset in any direction of the extreme upper end region of the associated post from a normally centered position wherein the post extends verti'cally. The lower mounting assembly 52 is centered about a hole 54 (see FIG. 5) which is formed in the associated angle bar 44 of the base frame 12. A tubular insert 56 is welded in position within the hole 54 so as to project both upwardly and downwardly above and below the hole, thus providing, in effect, upwardly and downwardly projecting boss-like protuberances. Up per and lower resilient sleeves 58 and 60, respectively, are mounted around and centered upon these protuberances on opposite sides of the angle bar 44 and the two sleeves are compressed against the angle bar and between upper and lower annular circular plates 62 and 64, respectively. The upper plate is welded to the outer tubular member 48 adjacent to the bottom or lower end, thereof and the lower plate 64 is received over and welded to the lower end region of an elongated vertically extending tie rod 66 which passes completely and loosely through the outer tubular member 48 and tubular insert 56. The extreme lower end of the outer tubular member 48 projects below the upper plate 62 and enters the upper end of the sleeve 58. A clamping nut 68 and a lock nut 70 serve to retain the lower plate 64 in position against the underneath face of the resilient sleeve 60. The two resilient sleeves 58 and 60 may be formed of any suitable cushioning material. Preferably they are formed of an elastorneric material, such as rubber, either natural or synthetic, or a rubber substitute. The upper mounting assembly 59 comprises a single resilient ela'stomeric or other flexible resilient and compressible sleeve 72. The latter is similar to the sleeves 58 and 60 and is seated within an inverted cup-shaped member 74 having a rim flange 76 which is welded between a pair of spaced apart webs or plates 78 on the outside face of the hopper 24.

The sleeve 72 is adapted to be compressed between the horizontal end wall of the cup-shaped member 74 and an annular plate Stl which is received over, and welded to, the outer tubular member 48. The end wall of the cupshaped member 74 is formed with a hole 82 therethrough and the upper end of the tubular member 48 extends into theresilient sleeve 72 and terminates below the plane 'of the end wall of the cup-shaped member 74. The upper end region of the tie rod 65 projects upwardly through the hole 82 and threadedly receives thereon a clamping nut 8 which effectively bears by reason of a metal washer 86 against a rubber or other elastomeric resilient washer From the above description it will be seen that the lower resilient mounting assemblies 52 are of such character that they will permit limited swivelling movement of the posts 46, the elastomeric material of the sleeves 58 and 60 yielding as the posts move in one direction or the other. Similarly, the upper resilient mounting assemblies 50 are of such character that they will permit limited lateral displacement of the upper end regions of the posts in any direction, the elastomeric sleeves 72 yielding as such displacement takes place. The normal position of the posts 46, however, is vertical.

Referring now to FIG. 6, the gyrating mechanism 14 whereby gyratory motion may be applied to the screen assembly includes a substantially vertically disposed tubular drive shaft 100 the lower end of which is rotatably supported by means of a combined thrust and roller bearing assembly 102 on a centering and supporting hub 104. The hub 104 is formed of resilient elastomeric material and is telescopically received over a flanged stud 106 (see FIG. 4). Such stud is bolted as at 108 to a crossbar 110 which extends across and is connected to the base frame 12. A metal reinforcing sleeve or liner is disposed within the elastomeric hub 104 and terminates below the upper end thereof. The hub 104 is adapted to be compressed by means of a metal washer 112 which is drawn against the upper end of the hub by a nut 114. Such compression of the hub causes the upper region thereof to become flanged over the upper rim of the bearing assembly 102 as indicated at 116 to retain the hub against vertical shifting or displacement. A washer 118 underlies the bearing assembly 102 and seats on an upwardly facing annular shoulder 119 on the hub 104. The nut 114, when tightened on the upper end of the stud 106 may be secured in position by means of a cotter pin 120. The upper end of the tubular drive shaft 100 is rotatably connected to the screen assembly 10 and, accordingly, a flanged stud 122 is welded to said upper end and receives thereover an elastomeric hub 124 which is similar to the hub 104. The hub 124 is rotatable with the drive shaft 100 and extends into a generally circular socket 126 in a cup-shaped centering cap 128. The latter is bolted as at 130 to the underneath side of a crossbar 132 which extends across and is fixedly connected to the sides of the hopper 24. A combined thrust and roller bearing assembly 134 surrounds the elastorneric hub 124 and seats on an upwardly facing shoulder, 136, the assembly 134 fitting tightly Within the centering cap 128. A washer 138 is received over the upper end of the stud 122 and serves to compress the elastomeric hub 124 un der the influence of a clamp bolt 140 threadedly received on the stud. When thus compressed, the material of the hub 124 extends over the upper rim of the bearing assembly 134 as indicated at 142. Secured as by welding to the extreme upper end region of the tubular drive shaft 159 is an eccentric weight 150 (see FIGS. 3 and 6). The weight is generally semi-circular in its configuration. It is recessed as at 152 so that it may fit against and closely hug the outer cylindrical face of the drive shaft. An

alignment pin 154 projects radially through the shaft and weight and facilitates the welding installation procedure.

viously mentioned power-actuated driving means 16 comprises an electric motor 160 which is suitably supported on the base frame 12 as, for example, by means of a mounting bracket 162 (see FIG. 2). The motor shaft 164 carries a driving pulley 166 which is operatively connected by an endless belt 167 to a driven pulley 168. The latter is welded or otherwise fixedly secured to the lower end of the tubular drive shaft 100.

In the operation of the herein described screening apparatus and, during the actual screening of sand, the electric motor 160 transmits rotary motion'to the tubular drive shaft 100 through the medium of the previously mentioned belt and pulley connections. Normally, the drive shaft 100 is maintained in a vertical position under the restraining influence of the four posts 46 which are maintained vertical as preivously described and are operatively connected to the drive shaft 100 through the inside marginal frame 28 of the hopper, the crossbar 132, the centering cap 128, and the various other elements of the resilient connection at the upper end of the drive shaft. However, the off-center eccentric weight 150 which is fixedly secured to and rotates with the drive shaft 100 has a relatively large amount of inertia and, since its center of mass is offset appreciably from the normal axis of rotation of the shaft, the weight 150 sets up a gyratory or wobbling motion in the upper end of the shaft. This gyratory motion is transmitted to a large degree through the resilient hub 124 to the hopper 24, and, consequently, to the screen assembly 10 as a whole. The screen assembly 10 is thus constrained to gyrate to a degree commensurate with the radius of gyration of the eifective mass which is attached to the upper end of the drive shaft 100 and includes not only the weight 150, the bearing support therefor, and the entire screen assembly 100, but also the elfective weight of any sand load which is placed or disposed on the screen assembly.

The term radius of gyration as used herein is not to be confused with the amplitude at which the upper end of the shaft 100 becomes displaced when the latter is rotated. Such amplitude is a variable factor and among the variables associated therewith are the speed or rotation of the shaft 100, the moment of inertia of the overall load involved, the eccentricity of the center of mass of such load, etc.

The radius of gyration is a constant for all speeds of rotation of the shaft and it has a fixed value which may be expressed by the equation where K is the radius of gyration, I is the moment'of inertia and M is the mass of the load. This radius of gyration in the present instance is the distance from the axis of the drive shaft 100 at which the entire mass of the screen assembly and its load may be considered concentrated.

The amplitude of gyration, i.e., the amount of displacement of the upper end of the drive shaft at any given instant during the gyratory motion thereof is dependant upon the radius of gyration, as well as upon the speed of rotation of the drive shaft 100. If the shaft 100 be considered to have a constant speed of rotation, then the amplitude of gyration will vary in direct pl'oportion to the radius of gyration. This radius of gyraupper end of the shaft under the restraining influence of tion will vary only insofar as the load on the screen varies. Ordinarily, another factor which would vary the radius of gyration of the upper end of the shaft would be the position of the sand load upon the upper surface of the screen, a concentration of sand in the outer portions of the screen bringing about a greater radius of gyration. However, since the screen is restrained from taking on any rotary motion and is confined solely to an oscillatory motion without rotation, the distribution of the sand on the surface of the screen may be disregarded and a load shift will not affect either the radius of gyration or the amplitude of gyration. The only factor, therefore, which will affect the amplitude of gyrationis theload which is placed upon the screen.

eral, the amplitude of gyration of the screen. will be in more or less inverse proportion to' the mass of the load upon. the screen. For example, with an empty or unloaded screen, maximum amplitude of gyration will be in effect whereas, with a fully loaded screen, minimum amplitude of gyration will obtain.

From the above description, it'willi be appreciated that, in the manufacture of the present screening apparatus, the eccentricweight 150 will be designed in accordance with the radius of gyration of the unloaded screen assembly 10, taking also into consideration. the predetermined speed of rotation of the shaft so that the amplitude of gyration of the screen assembly 10,. when operating in the absence of a load, will not be excessive and will not set up vibrational elfects which are objectionable from the noise level point of view; Then, as the load is placed upon the screen, the tendency will be for a reduction in the amplitude of gyration, such reduction taking place progressively up to the placing of a maximum sand load upon the screen. The centrifugal effect of the eccentric weight 156 is thus variable within: the limits of the load imposed upon the screen and, to the extent that some of this centrifugal effect is'cancelled out with each additional increment of load. upon the screen, the eccentric weight may be said to be self-balancing.

It is'to be noted that due to the fact that, unlike conventional dual drive shaft arrangements wherein the screen assembly is positively guided in a fixed eccentric orbit in which the amplitude of gyration does not vary, an eccentric weight of a predetermined mass will suffice for a wide variance in the san load and need not be replaced with a weight of different mass each time the load is changed, Such an eccentric weight, furthermore, will adjust itself to conditions as the sand runs out and, at the time the last sand particle has been discharged from the surface of the screen, the maximum vibration which will take place will be well within precalculated safe and unobjectionable limits. Finally, due to the fact that the lower end region of the drive shaft 1% ismaintained on a substantially fixed center, constant belt tension is encountered in the belt drive mechanism whichconstitutes a part of the power-actuated driving means 16. Belt life is "thus prolonged and the motor 160 is thus protected from vibrational effects. Generally speaking,

all vibrational effects are confined to the gyrating screen assembly 10 and are excluded from the lower base frame 12 and other stationary portions of the apparatus.

The invention is not to be limited to the exact ar- 'rangement of parts shown in the accompanying drawings or described in this specification as various changes in 'the details of construction may be resorted to without departing from the spirit or scope of the invention. For example, the screen proper 2t) shown herein is of the conventional sandwich type but other forms of screens are contemplated as, for example, an undulatory type of screen such as has been disclosed in United States base frame and positioned above the latter, means ex- 8 tending between. the base frame and screen frame for supporting the latter from. the former so that it is free to gyrate in a substantially horizontal plane at different amplitudes about alfixecl center while normally and yieldingly biasing the same toward. such fixed center, and means for effecting. gryration of said. movable screen frame, said. means comprising a; stud fixed to and projecting upwardly from the base frame, a resilient elastomeric supporting hubtelescopically received over said stud, a; bearing assembly including an inner race, an outer race and interposed rolling elements between said races and telescopically received over said hub, the upper end of the stud being threaded, aclamping nut threadedly' received on the upper end of. the stud and serving to compress the material of the hub, to flange the upper end' of the hub over the inner race of the bearing. assembly, and to' expand the hub radially outwardly so as to seize the inner race and maintain the bearing assembly floatingly supported on the hub, a vertically disposed tubular shaft having its lower open rim secured to and rotatable with said outer race, a drive pulley mounted on said shaft in coaxial relationship with respect thereto and surrounding the bearing assembly at the same horizontal level, means rotatably journalling the upper end of said shaft in said screen frame whereby lateral displacements of said upper end will be transmitted to the screen frame bodily, said journalling means comprising a flanged stud closing the upper open rim of the tubular shaft andprojecting vertically upwardly therefrom, a second resilient clastomeric hub telescopically received over said flanged stud, a second bearing assembly including an inner race, an outer race and interposed rolling elements between said races and telescopically received over said second hub, the upper end of said second hub being threaded, a clamping nut threadedly received on the upper end of the second stud and serving to compress the material of the second hub, to flange the upper end thereof over the inner race of the second bearing assembly, and to expand the second hub radially outwardly so as to seize the inner race and maintain the second bearing assembly floatingly supported on the second hub, and a centering cap fixedly secured to the outer race of the second bearing assembly and to the screen frame, and an eccentric weight fixedly secured to said tubular shaft adjacent to the upper end thereof and having its center of mass displaced outwardly of the shaft axis so that upon rotation of the shaft the upper end of the shaft will be constrained to gyratc under the influence of centrifugal forces acting upon said upper end, thereby transmitting gyrational effects to the screen frame through the centering cap.

2. A screening apparatus comprising, in combination, a fixed base frame, a movable screen frame including a substantially horizontally disposed screen disposed over and in vertically spaced relation with respect to said base frame, means'extending between the base frame and screen frame for supporting the latter from the former so that it is free to gyrate in a substantially hori zontal plane at different amplitudes about a fixed center while normally and yieldin ly biasing the same toward such fixed center, and means for effecting gyration of said movable screen frame in the horizontal plane thereof, said means comprisin an upstanding elongated rigid rotary drive shaft extending between the base frame and the screen frame, means swivelly connecting the extreme lower end of said drive shaft to the base frame whereby the shaft is capable of limited tilting movements about its swivel point, means rotatably journalling the upper end of the drive shaft in said screen frame whereby lateral components of displacement of said upper end will be transmitted to the screen frame bodily, said journalling means comprising a roller bearing having inner and outer races, means securing the outer race to said screen frame, the upper end of said drive shaft projecting into the inner race, and an elastomeric spacing sleeve interposed between the upper end of the shaft and the inner race, said elastomeric sleeve allowing for limited relative angular and vertical lost motion displacements between the shaft and the screen frame, an eccentric weight fixedly secured to said shaft adjacent to the upper end thereof and having its center of mass displaced outward- 1y of the shaft axis so that upon rotation of the shaft the upper end thereof will be constrained to gyrate under the influence of centrifugal forces acting upon said up-,

per end, thereby transmitting gyrational effects to the screen frame through said elastomeric spacing sleeve, and means for applying torque to the extreme lower end of said drive shaft to effect rotation of the same.

3. A screening apparaus as set forth in claim 2 and wherein said means for applying torque to the extreme lower end of the drive shaft comprises a driving motor mounted on said base frame and having a motor shaft,

a driven pulley on said drive shaft in the horizontal plane of the swivel connection between the shaft and 10 base frame, a driving pulley on the motor shaft, and a flexible belt passing around said driving and driven pulleys.

References Cited in the file of this patent UNITED STATES PATENTS 546,241 McAnulty Sept. 19, 1895 1,866,531 Harding July 12, 1932 2,194,721 Piper Mar. -26, 1940 2,238,435 erry Apr. 15, 1941 2,255,799 Meinzer Sept. 16, 1941 2,305,344 Gary Dec. 15, 1942 2,572,265 Johnson Oct. 23, 1951 2,682,338 Hurst June 29, 1954 2,950,819 Holman Aug. 30, 1960 FOREIGN PATENTS 1,218,627 France Dec. 21, 1959 

2. A SCREENING APPARATUS COMPRISING, IN COMBINATION, A FIXED BASE FRAME, A MOVABLE SCREEN FRAME INCLUDING A SUBSTANTIALLY HORIZONTALLY DISPOSED SCREEN DISPOSED OVER AND IN VERTICALLY SPACED RELATION WITH RESPECT TO SAID BASE FRAME, MEANS EXTENDING BETWEEN THE BASE FRAME AND SCREEN FRAME FOR SUPPORTING THE LATTER FROM THE FORMER SO THAT IT IS FREE TO GYRATE IN A SUBSTANTIALLY HORIZONTAL PLANE AT DIFFERENT AMPLITUDES ABOUT A FIXED CENTER WHILE NORMALLY AND YIELDINGLY BIASING THE SAME TOWARD SUCH FIXED CENTER, AND MEANS FOR EFFECTING GYRATION OF SAID MOVABLE SCREEN FRAME IN THE HORIZONTAL PLANE THEREOF, SAID MEANS COMPRISING AN UPSTANDING ELONGATED RIGID ROTARY DRIVE SHAFT EXTENDING BETWEEN THE BASE FRAME AND THE SCREEN FRAME, MEANS SWIVELLY CONNECTING THE EXTREME LOWER END OF SAID DRIVE SHAFT TO THE BASE FRAME WHEREBY THE SHAFT IS CAPABLE OF LIMITED TILTING MOVEMENTS ABOUT ITS SWIVEL POINT, MEANS ROTATABLY JOURNALLING THE UPPER END OF THE DRIVE SHAFT IN SAID SCREEN FRAME WHEREBY LATERAL COMPONENTS OF DISPLACEMENT OF SAID UPPER END WILL BE TRANSMITTED TO THE SCREEN FRAME BODILY, SAID JOURNALLING MEANS COMPRISING A ROLLER BEARING HAVING INNER AND OUTER RACES, MEANS SECURING THE OUTER RACE TO SAID SCREEN FRAME, THE UPPER END OF SAID DRIVE SHAFT PROJECTING INTO THE INNER RACE, AND AN ELASTOMERIC SPACING SLEEVE INTERPOSED BETWEEN THE UPPER END OF THE SHAFT AND THE INNER RACE, SAID ELASTOMERIC SLEEVE ALLOWING FOR LIMITED RELATIVE ANGULAR AND VERTICAL LOST MOTION DISPLACEMENTS BETWEEN THE SHAFT AND THE SCREEN FRAME, AN ECCENTRIC WEIGHT FIXEDLY SECURED TO SAID SHAFT ADJACENT TO THE UPPER END THEREOF AND HAVING ITS CENTER OF MASS DISPLACED OUTWARDLY OF THE SHAFT AXIS SO THAT UPON ROTATION OF THE SHAFT THE UPPER END THEREOF WILL BE CONSTRAINED TO GYRATE UNDER THE INFLUENCE OF CENTRIFUGAL FORCES ACTING UPON SAID UPPER END, THEREBY TRANSMITTING GYRATIONAL EFFECTS TO THE SCREEN FRAME THROUGH SAID ELASTOMERIC SPACING SLEEVE, AND MEANS FOR APPLYING TORQUE TO THE EXTREME LOWER END OF SAID DRIVE SHAFT TO EFFECT ROTATION OF THE SAME. 